A mixed-mode failure criterion derived from tests on symmetric and asymmetric specimens

Delamination experiments were first performed on unidirectional glass/epoxy composite specimens to determine a mixed-mode initiation failure criterion. The tests employed use the double cantilever beam (DCB) specimen for mode I, end-notched flexure (ENF) for mode II and mixed-mode bending (MMB) for I/II mixed-mode loading. MMB experiments are analysed according to a method based on beam theory and an experimental method which uses measured displacements. These two analyses give similar results. The initiation failure point is defined both by the first non-linearity of the load/displacement curve and by use of the first acoustic signal. These two definitions result in values very close to each other. A validation of this criterion is then carried out on asymmetric DCB and MMB specimens of the same material. Mixed-mode partitioning is performed by three different methods. These are the global and local methods, which are based on analytical expressions, and the modified crack-closure integral (MCCI), which uses a finite-element analysis. The three failure envelopes obtained are compared with the failure criterion determined on symmetric specimens. A very good correlation is observed between the mixed-mode results from symmetric specimens and the failure envelopes determined by both the local and finite-element methods of mode partitioning for asymmetric specimens. On the other hand, the global method does not seem to be well adapted to such tests for this material.